Re: [PATCH] memcg: restore ss->id_lock to spinlock, using RCU for next

Hugh Dickins wrote:> Commit c1e2ee2dc436 "memcg: replace ss->id_lock with a rwlock" has> now been seen to cause the unfair behavior we should have expected> from converting a spinlock to an rwlock: softlockup in cgroup_mkdir(),> whose get_new_cssid() is waiting for the wlock, while there are 19> tasks using the rlock in css_get_next() to get on with their memcg> workload (in an artificial test, admittedly). Yet lib/idr.c was> made suitable for RCU way back.> > 1. Revert that commit, restoring ss->id_lock to a spinlock.> > 2. Make one small adjustment to idr_get_next(): take the height from> the top layer (stable under RCU) instead of from the root (unprotected> by RCU), as idr_find() does.> > 3. Remove lock and unlock around css_get_next()'s call to idr_get_next():> memcg iterators (only users of css_get_next) already did rcu_read_lock(),> and comment demands that, but add a WARN_ON_ONCE to make sure of it.> > Signed-off-by: Hugh Dickins <hughd@google.com>